What sink, what kind of faucet?
Yes you have to turn off the water. Usally under the sink are shutoff valves. They can work or not. Be sure to check after you turn them off. Looking at the valve handle, clockwise is off.
Once it is off, remove the handle (this is why I need to know the type of faucet – Single handle, two, what brand etc. as they are all a little different.
Feel free to email if you can’t or haven’t taken care of it.
Tom Saggau